1. Field of the Invention
The present invention relates to the papermaking arts. More specifically, the present invention relates to a leader harness used to pull a seamable papermaker's fabric onto a paper machine.
2. Description of the Prior Art
During the papermaking process, a cellulosic fibrous web is formed by depositing a fibrous slurry, that is, an aqueous dispersion of cellulose fibers, onto a moving forming fabric in the forming section of a paper machine. A large amount of water is drained from the slurry through the forming fabric, leaving the cellulosic fibrous web on the surface of the forming fabric.
The newly formed cellulosic fibrous web proceeds from the forming section to a press section, which includes a series of press nips. The cellulosic fibrous web passes through the press nips supported by a press fabric, or, as is often the case, between two such press fabrics. In the press nips, the cellulosic fibrous web is subjected to compressive forces which squeeze water therefrom, and which adhere the cellulosic fibers in the web to one another to turn the cellulosic fibrous web into a paper sheet. The water is accepted by the press fabric or fabrics and, ideally, does not return to the paper sheet.
The paper sheet finally proceeds to a dryer section, which includes at least one series of rotatable dryer drums or cylinders, which are internally heated by steam. The newly formed paper sheet is directed in a serpentine path sequentially around each in the series of drums by a dryer fabric, which holds the paper sheet closely against the surfaces of the drums. The heated drums reduce the water content of the paper sheet to a desirable level through evaporation.
It should be appreciated that the forming, press and dryer fabrics all take the form of endless loops on the paper machine and function in the manner of conveyors. It should further be appreciated that paper manufacture is a continuous process which proceeds at considerable speeds. That is to say, the fibrous slurry is continuously deposited onto the forming fabric in the forming section, while a newly manufactured paper sheet is continuously wound onto rolls after it exits from the dryer section.
As implied above, forming fabrics function to form and convey the paper product being manufactured to the press section. However, forming fabrics also need to address water removal and sheet formation issues. That is, forming fabrics are designed to allow water to pass through (i.e. control the rate of drainage) while at the same time prevent fiber and other solids from passing through with the water. If drainage occurs too rapidly or too slowly, the sheet quality and machine efficiency suffers. To control drainage, the space within the forming fabric for the water to drain, commonly referred to as void volume, must be properly designed.
Press fabrics also participate in the finishing of the surface of the paper sheet. That is, press fabrics are designed to have smooth surfaces and uniformly resilient structures, so that, in the course of passing through the press nips, a smooth, mark-free surface is imparted to the paper. Press fabrics accept the large quantities of water extracted from the wet paper in the press nip. Hence, void volume is also important in press fabrics to provide a path for the water to go. The fabric must also have adequate permeability to water for its entire useful life. Finally, press fabrics must be able to prevent the water accepted from the wet paper from returning to and rewetting the paper upon exit from the press nip.
Woven fabrics take many different forms. For example, they may be woven endless, or flat woven and subsequently rendered into endless form with a seam. Woven fabrics are typically in the form of endless loops, or are seamable into such forms, having a specific length, measured longitudinally therearound, and a specific width, measured transversely thereacross. Because paper machine configurations vary widely, paper machine clothing manufacturers are required to produce fabrics, and other paper machine clothing, to the dimensions required to fit particular positions in the paper machines of their customers. Needless to say, this requirement makes it difficult to streamline the manufacturing process, as each fabric must typically be made to order.
Fabrics in modern papermaking machines may have a width of from 5 to over 33 feet, a length of from 40 to over 400 feet and weigh from approximately 100 to over 3,000 pounds. These fabrics wear out and require replacement. Replacement of fabrics often involves taking the machine out of service, removing the worn fabric, setting up to install a fabric and installing the new fabric. While many fabrics are endless, about half of those used in press sections of the paper machines today are on-machine-seamable. Some Paper Industry Process Belts (PIPBs) are contemplated to have an on machine seam capability, such as some transfer belts, known as Transbelt®. Installation of the fabric includes pulling the fabric body onto a machine and joining the fabric ends to form an endless belt. Almost all dryer fabrics today have some type of seam.
An important aspect of loading a fabric body onto a paper machine is that there be uniform tension across the fabric. If uniform tension is not achieved and one section of the fabric pulls more than another, then the fabric can bubble or ridge across the fabric width.
Another aspect of loading a fabric body is preventing damage to the fabric body seam. In order to avoid or minimize the chance of damage to the seam during installation, uneven tension, weight and pressure must be avoided on the seam itself.
It has been common practice to attach zippers and Velcro-type leaders to fabrics by use of staples, sewing and/or adhesive materials. However, since these attachment methods can damage the fabric surface, it is preferable to use methods which do not damage the fabric.
A further aspect of loading a fabric, especially very long ones is properly aligning the fabric body in the machine so the fabric guides true in the machine direction (MD) and does not oscillate or track to one side of the machine. If the fabric guides or tracks poorly it can make contact with the paper machine support frame and cause fabric damage.
For fabrics and belts with seams that can be joined together on the paper machine, various types of leaders have been tried to assist installation. In order to avoid or minimize the potential for damaging the fabric body and the machine during installation and operation, the leader should be designed so there is uniform tension across the fabric body. There have been several attempts to design such leaders.
U.S. Pat. Nos. 5,306,393 and 5,429,719 both to Rhyne describe a device and method for installing a fabric body onto a paper machine. The method includes providing a self-aligning fabric loading harness having a leading edge and a plurality of spaced empty grommets disposed adjacent to the leading edge, to which multiple ropes are attached, securing a pull rope through a loading harness and a line receiving device, pulling the pull rope, and automatically readjusting the pull rope through the loading harness to attempt to achieve uniform tension across the fabric.
Some leaders are square or rectangular, with the long dimension in either the machine direction (MD) or cross-machine direction (CD). Multiple ropes or straps are attached to the leader at evenly spaced locations across the width of the leader. The leader with the attached papermaker's fabric or belt is pulled through the fabric run. The ends of the papermaker's fabric or belt are brought together and joined by a seam to make the fabric endless. The leader is removed and the fabric is ready for use. However, the multiple ropes or straps can get hung up on stationary equipment in the fabric run, causing a difficult and time consuming installation, if not tearing and damaging the fabric.
There are also leaders currently used in the industry which are shaped like an isosceles triangle, having the apex removed to form a trapezoid. The leaders are typically fabricated from a woven material, but the material can also be nonwoven. The base of a leader has a zipper, which is used to attach the leader to an end of the fabric being installed on the paper machine. Such a design is preferred because only one rope is attached near the apex to pull the fabric onto the machine. When the triangle is cut from woven material, one of the yarn systems in the weave goes straight from the base to the apex and the other is at a 90 degree angle thereto.
FIG. 1 shows a top view of a prior art leader 10. Leader 10 is shaped like an isosceles triangle and is fabricated from a woven material. The base 12 of leader 10 has one half of a zipper along its edge 12, which is used to attach leader 10 to an end of the fabric being installed on the paper machine to which the other half of the zipper is attached to the fabric or belt. Papermill personnel can attach a rope near the apex which is provided with a hole 14 and pull the fabric onto the machine. When the triangle is cut from woven material, one of the yarn systems in the weave goes straight from the base to the apex and the other is at a 90 degree angle thereto. When the rope is pulled as shown in FIG. 1a, the force is unevenly distributed across the leader as well as the attached fabric 16 which causes the fabric body to bunch on the sides 18.
Full width steel bars may be inserted at the base of the leader for better weight/tension distribution. However, the bars are heavy, thick and sometimes difficult to pass through the nip formed by two press rolls, or a shoe and opposing roll.
With a leader of this type, even with a 4 foot wide (in the cross machine direction or CD) steel reinforcing bar at the apex of the triangle/trapezoid, when the rope is pulled the force is unevenly distributed about the leader and across the attached fabric body. When the apex above is pulled, most of the force is distributed over the center four feet of the leader. This causes the center of the fabric to bunch up, making it more difficult to seam, and often causes the edges of the fabric or belt 16 and leader 10 to droop 18 and 20 while being pulled onto the paper machine.
The drawback of this type of leader is that the load is always concentrated down its center. This causes both the center of the leader and the fabric attached to it, to lead the edges and form waves in the center while pulling through the machine, making it more difficult to seam as well as guide the fabric through the run during installation. This often causes the edges of the fabric to droop while it is being pulled through the fabric run. Any fabric edge droop or bunching/waviness (i.e. any departure from a relatively flat fabric profile) can cause the fabric to become hung up on stationary equipment, or to not easily pass through the gap formed between two press rolls. Attempts to correct both the fabric and leader edges from drooping by inserting ropes down the edges, usually results in the edges curling up and folding over, which is also not desirable.
While the aforementioned types of methods and devices for installing an on machine seamable fabric or belt have particular advantages, they also have attendant disadvantages as discussed above.